Positive Electrode Active Material, Non-Aqueous Electrolyte Secondary Battery and Method for Producing Positive Electrode Active Material
1. Technical Field
This invention relates to a method for producing a positive electrode active material that is capable of reversibly doping/undoping lithium, and a method for producing a non-aqueous electrolyte secondary battery employing this positive electrode active material.
2. Background Art
Recently, with the marked progress in a variety of electronic equipment, researches in a rechargeable secondary battery, as a battery that can be used conveniently and economically for prolonged time, are underway. Typical of the known secondary batteries are a lead battery, an alkali storage battery and a lithium secondary battery.
Of these secondary batteries, a lithium secondary battery has advantages in high output and in high energy density. The lithium secondary battery is made up at least of positive and negative electrodes, containing active materials capable of reversibly introducing and removing lithium ions, and a non-aqueous electrolyte.
Nowadays, a compound having an olivinic structure, such as, for example, a compound represented by a general formula LixMyPO4, where x is such that 0 less than xxe2x89xa62 and y is such that 0.8xe2x89xa6yxe2x89xa61.2, with M containing a 3d transition metal, is retained to be promising as a positive electrode active material for a lithium secondary battery.
It has been proposed in Japanese Laying-Open Patent H-9-171827 to use e.g., LiFePO4, among the compounds represented by LixMyPO4, as a positive electrode for a lithium ion battery.
LiFePO4 has a theoretical capacity as high as 170 mAh/g and, in an initial state, contains electro-chemically dopable Li per Fe atom, so that it is a material promising as a positive electrode active material for a lithium ion battery.
Up to now, LiFePO4 was synthesized using a salt of bivalent iron, such as iron acetate Fe(CH3COO)2, as a source of Fe as a starting material for synthesis, and on sintering the starting material at a higher temperature of 800xc2x0 C. under a reducing atmosphere.
However, it is reported in the above publication that, in the battery prepared using LiFePO4, prepared by the above method for synthesis, as the positive electrode active material, the real capacity only on the order of 60 mAh/g to 70 mAh/g may be realized. Although the real capacity of the order of 120 mAh/g has been reported in Journal of the Electrochemical Society, 144, 1188 (1997), this real capacity cannot be said to be sufficient in consideration that the theoretical capacity is 170 mAh/g.
If LiFePO4 is compared to LiMn2O4, LiFePO4 has a volumetric density and an average voltage of 3.6 g/cm2 and 3.4 V, respectively, whereas LiMnPO4 has a volumetric density and an average voltage of 4.2 g/cm2 and 3.9 V, respectively, with its capacity being 120 mAh/g. So, LiFePO4 is smaller by approximately 10% in both the voltage and the volumetric density than LiMn2O4. So, with the same capacity if 120 mAh/g, LiFePO4 is smaller than LiMn2O4 by not less than 10% in weight energy density and by not less than 20% in volumetric energy density. Thus, for realizing an equivalent or higher level in LiFePO4 with respect to LiMn2PO4, a capacity equal to or higher than 140 mAh/g, is required, however, such a high capacity has not been achieved with LiFePO4.
On the other hand, with LiFePO4, synthesized on sintering at a higher temperature of 800xc2x0 C., there are occasions where crystallization proceeds excessively to retard lithium diffusion. So, with the non-aqueous electrolyte secondary battery, sufficiently high capacity has not been achieved. Moreover, if the sintering temperature is high, the energy consumption is correspondingly increased, while a higher load is imposed on e.g., a reaction apparatus.
It is an object of the present invention to provide a positive electrode active material which realizes a high capacity if used in a battery, and a non-aqueous electrolyte secondary battery employing the positive electrode active material.
For accomplishing the above object, the present invention provides a positive electrode active material containing a compound represented by the general formula LixMyPO4, where 0 less than xxe2x89xa62 and 0.8xe2x89xa6yxe2x89xa61.2, with M containing a 3d transition metal, where the LixMyPO4 encompasses that with the grain size not larger than 10 xcexcm.
The positive electrode active material according to the present invention contains LixMyPO4 with the grain size not larger than 10 xcexcm. In this manner, the positive electrode active material is of a grain size distribution enabling e.g., lithium, as charge carrier, to be diffused sufficiently in the grains of the positive electrode active material.
The present invention also provides a positive electrode active material containing a compound represented by the general formula Lix(FeyM1xe2x88x92y)PO4, where 0.9xe2x89xa6xxe2x89xa61.1 and 0 less than yxe2x89xa61, with M containing a 3d transition metal, wherein, in a spectrum for the Lix(FeyM1xe2x88x92y)PO4 obtained by the Moessbauer spectroscopic method, A/B is less than 0.3, where A is the area strength of a spectrum obtained by the Moessbauer spectroscopic method of not less than 0.1 mm/sec and not larger than 0.7 mm/sec and B is the area strength of a spectrum obtained by the Moessbauer spectroscopic method not less than 0.8 mm/sec and not larger than 1.5 mm/sec.
With this positive electrode active material, according to the present invention, since A/B is less than 0.3, the quantity of electrochemically inert impurities is small, thus realizing a high capacity.
The present invention also provides a non-aqueous electrolyte secondary battery including a positive electrode having a positive electrode active material containing a compound represented by the general formula LixMyPO4, where 0 less than xxe2x89xa62 and 0.8xe2x89xa6yxe2x89xa61.2, with M containing a 3d transition metal, a negative electrode having a negative electrode active material, the positive electrode active material and the negative electrode active material being capable of reversibly doping/undoping lithium, and a non-aqueous electrolyte, wherein the LixMyPO4 encompasses that with the grain size not larger than 10 xcexcm.
The non-aqueous electrolyte secondary battery according to the present invention contains LixMyPO4, with the grain size not larger than 10 xcexcm, as a positive electrode active material. This positive electrode active material is of such a grain size distribution that enables lithium as a charge carrier to be diffused sufficiently in the grains. Thus, the non-aqueous electrolyte secondary battery is of high capacity.
The present invention also provides a non-aqueous electrolyte secondary battery including a positive electrode having a positive electrode active material containing a compound represented by the general formula Lix(FeyM1xe2x88x92y)PO4, where 0.9xe2x89xa6xxe2x89xa61.1 and 0 less than yxe2x89xa61, with M containing a 3d transition metal, a negative electrode having a negative electrode active material, the positive electrode active material and the negative electrode active material being capable of reversibly doping/undoping lithium, and a non-aqueous electrolyte, wherein, in a spectrum for the Lix(FeyM1xe2x88x92y)PO4 obtained by the Moessbauer spectroscopic method, A/B is less than 0.3, where A is the area strength of a spectrum obtained by the Moessbauer spectroscopic method not less than 0.1 mm/sec and not larger than 0.7 mm/sec and B is the area strength of a spectrum obtained by the Moessbauer spectroscopic method not less than 0.8 mm/sec and not larger than 1.5 mm/sec.
The non-aqueous electrolyte secondary battery, according to the present invention, is of the value of A/B less than 0.3, and contains the positive electrode active material with low content of electrochemically inert impurities, thus realizing a non-aqueous electrolyte secondary battery of a high capacity.
It is another object of the present invention to provide a method for producing a positive electrode active material which, if used in a battery, realizes a high battery capacity.
For accomplishing the above object, the present invention provides a method for producing a positive electrode active material including a mixing step of mixing a starting material for synthesis of a compound represented by the general formula LixMyPO4, where 0 less than xxe2x89xa62 and 0.8xe2x89xa6yxe2x89xa61.2, with M containing a 3d transition metal, and a sintering step of sintering and reacting the precursor obtained in the mixing step, wherein, in the sintering step, the precursor is sintered at a temperature not lower than 400xc2x0 C. and not higher than 700xc2x0 C.
In the manufacturing method for the positive electrode active material according to the present invention, the precursor of LixMyPO4 is sintered in the sintering step at a temperature not lower than 400xc2x0 C. and not higher than 700xc2x0 C. So, the chemical reaction and crystallization proceed uniformly, without the crystallization proceeding excessively, to yield impurity-free single-phase LixMyPO4. Also, the powder characteristics of LixMyPO4 are changed dramatically due to the difference in the temperature of sintering the precursor of LixMyPO4.